The phorbol diesters, e.g., phorbol-12-myristate-13-acetate (PMA), are extremely potent tumor promoters in mouse skin. In chick embryo fibroblasts (CEFs) in vitro, PMA at nanomolar concentrations induces a number of the same changes brought about by infection with Rous sarcoma virus (RSV). These changes include elevated production of plasminogen activator, stimulation of growth and deoxyglucose uptake, and decreased levels of the transformation-sensitive surface protein LETS. We propose to examine in detail the mode of action of PMA in CEFs. The studies have two objectives: 1) To shed light on the phenomenon of tumor promotion, and 2) To clarify the control mechanisms of transformation-sensitive cellular properties. The structure-activity relationships of phorbol derivatives in CEFs will be compared with their activities as inflammatory and tumor promoting agents in mouse skin. The stability of the derivatives under tissue culture conditions will be examined to determine the role of degradation in modifying these relations. The spectrum of biochemical changes induced by PMA and RSV will be compared to better determine the degree to which the two induced phenotypes resemble each other. Particular attention will be directed at the surface changes induced by PMA. In an attempt to identify PMA receptors, binding studies will be performed with 3H-PMA. If identified, attempts to purify the receptors will be undertaken. Screening will be done to detect potential analogs and antagonists of the phorbol derivatives. Cell lines other than CEFs will be examined for similar responses to PMA, and, if possible, variants of these cell lines resistant to the actions of PMA will be isolated.